This invention relates to co-ordinate measuring for inspecting the dimensions of workpieces.
It is common practice, after workpieces have been produced, to inspect them on a co-ordinate measuring machine (CMM) having a quill which can be driven in three orthogonal directions, X,Y,Z, within a working volume of the machine. The quill may carry a touch trigger probe of the type described in U.S. Pat. No. 4,153,998 (McMurtry) which produces a trigger signal when its stylus touches a surface of the workpiece being inspected. This trigger signal is used to freeze the readings of scales or resolvers which indicate the X,Y and Z positions of the quill of the CMM, to indicate the X,Y and Z co-ordinates of the position of contact. Commonly, the CMM includes a computer program to take a series of readings of co-ordinate data at various locations on the workpiece, in order to perform a complete inspection of desired dimensions of the workpiece. A determination can then be made as to whether the workpiece is of acceptable quality, or whether it should be rejected.
The probes used in such applications are capable of great repeatability. However, it is known that they suffer from pre-travel. This is an inaccuracy caused by the slight delay between the instant of stylus contact with the workpiece and the generation of the trigger signal. It is exacerbated by the unavoidable delay in the response time before occurrence of the trigger signal causes freezing of the scale outputs and the taking of a reading. However, since the probe operates in a repeatable manner, this inaccuracy will generally be constant for a given direction and speed of approach of the probe to the workpiece.
Another cause of inaccuracy lies in the CMM itself. As the probe-carrying quill is accelerated and decelerated in the three dimensions, the structure of the machine is subject to deflection under dynamic loadings. The amount of such deflection depends on the instantaneous direction and speed of probe movement, and on the instantaneous X,Y,Z position in the working volume of the machine.
The solution to these problems which has been adopted to date involves causing the probe to travel very slowly as it contacts the workpiece surface. This reduces inaccuracies caused by the dynamic deflections and the pre-travel of the probe. Prior to taking any measurements, it is also usual to include a calibration or "datuming" cycle, in which the probe is calibrated by touching surfaces of a reference object at the slow speed. This enables the calculation of offsets which are stored and used to correct subsequent measurement readings for such factors as the diameter of the probe stylus. If the reference object is touched in each direction of probing movement, such a datuming cycle can also correct for the probe pre-travel in each such direction, but not for the dynamic deflections which vary from place to place within the working volume.
However, the process of inspecting a workpiece would take a very long time if the probe always moved at such a slow speed. Accordingly, a technique which has been used involves moving the probe at a high speed until it contacts the workpiece surface. This gives an approximate indication of the location of the workpiece surface. The probe is then reversed out of contact, then moved back into contact at a slow speed to obtain an accurate reading of its position. Such a technique enables a workpiece to be inspected faster than if the probe were always moved at a slow speed, though obviously the double touching at each inspection location (once at a high speed and once at a low speed) does take up time.
U.S. Pat. No. 4,333,238 (McMurtry) shows an alternative solution, in which the dynamic deflections are determined by sensing the accelerations of the structure of the CMM. However, this requires that an accelerometer should be specially provided and calibrated to sense the accelerations.
In place of the trigger probes discussed above it is also known to scan the contour of a workpiece surface with a measurement probe (i.e. one which continually determines the position of the surface relative to the body of the probe). An example is seen in U.S. Pat. No. 4,158,919 (McMurtry). The analogue or digital output of such a probe is added to the X,Y,Z outputs of the CMM's scales or resolvers to give the position of the surface. Of course, if the structure of the CMM is subject to dynamic deflections, these deflections will cause inaccuracies, just as they do with touch trigger probes. This is increasingly a problem as equipment capable of higher and higher scanning speeds is developed.